As is known, in the field of chemical analysis of body fluids, particularly by means of automatic analytical tools, there exists the particular need to have test tubes, pipettes or other like containers with containment side walls which are perfectly aligned with the longitudinal extension axis of the test tube itself and have a constant thickness along the entire axis.
This need is particularly important, for example, in the tests for measuring a blood sample's erythrocyte sedimentation rate (ESR). As is known, in fact, the erythrocyte sedimentation process is strongly influenced by the shape of the side walls of the test tube which is used to contain the blood sample to be analysed. In fact, if the test tube has an internal cross-section which is not perfectly constant and, thus, internal walls not perfectly aligned relative to its longitudinal extension axis, the erythrocytes inevitably tend to deposit themselves on the internal walls of the test tube itself, thus slowing their sedimentation movement toward the bottom. The ESR values obtained from measurements carried out in such test tubes shall result, therefore, misrepresented and unreliable. In the tests for measuring the ESR the need to have test tubes with side walls having a perfectly constant thickness is not connected per se to the process of erythrosedimentation, but to the appearance of optical type, automatic measuring tools. These tools are calibrated depending on the thickness of the test tube's side walls, since the measurement they give also depends on the optical path of the reading rays through the walls. Therefore, possible irregularities in the thickness of the walls, by modifying the optical path of the rays, can cause the automatic reading tool to provide measurement values outside the calibration range and therefore unreliable.
As is known, the current processes for the injection molding of plastic materials do not allow for making laboratory test tubes which have all the constructive peculiarities stated above, that is perfectly vertical side walls and constant thickness along the entire longitudinal extension of the test tubes themselves. Therefore, traditionally, to produce such test tubes, glass has always been used, which, with respect to plastic materials, can be processed with operatively more flexible molding processes and which is especially capable of providing final products with extreme dimensional tolerances.
The impossibility to use plastic materials in the production of this type of test tubes derives, firstly, from the operative difficulties encountered in keeping the male perfectly aligned and centred within the mold during the molding process and, secondly, from the difficulties encountered in expelling air from the mold during the injection step of the melted plastic material.
More in detail, the difficulties of centring the male within the mold are a consequence of the fact that the dimensions of the male are bound to those of the test tubes to be made (internal diameter of about 6-7 mm, wall thickness of about 1 mm and length of about 11-12 cm). The male is particularly slender and therefore it is not sufficiently strong and rigid to stand the high molding pressures requested (in the order of 100 bar) in the case of the molding of plastic materials without undergoing bending relative to the central axis of the mold. This would inevitably lead to have plastic test tubes with inclined side walls and with non-constant thickness.
This problem is emphasized, moreover, by the fact that, in order to ease the expulsion of all the air present in the mold, the melted plastic material is injected in the mold preferably at the bottom of the test tube. In fact, with an injection from the bottom, the air is pushed toward the mouth of the test tube where it can easily come out without special air-expellers. Therefore, there is the advantage of having a mold which is constructively simple to make and operatively reliable. However, in this way, the injection pressures of higher intensity are exerted just at the free end of the male, that is in the area where the latter is less rigid and is more easily subject to bending.
To limit the bending of the male, the plastic material can be injected in the mold at the mouth or possibly along the longitudinal extension axis of the test tube. With this solution, the injection pressures are exerted in areas where the male is more rigid. However, the air contained inside the mold is pushed, at least partially, in the molding area corresponding to the bottom of the test tube. Therefore, it necessary to provide, in the mold, a set of expellers to allow the evacuation of the air and prevent it from being trapped as bubbles inside the plastic matrix. In fact, considering the reduced thicknesses of the test tube's walls, the presence of air bubbles could generate micro pores capable of compromising the impermeability of the test tube, which would then become totally unusable.
From an operative point of view, this second solution requires therefore providing a constructively much more complicated mold compared to the one requested for the injection from the bottom. Moreover, this second solution, even though it partially solves the problem of the centring of the male, is quite unreliable. In fact, it is known in the art that the air expellers currently used are frequently obstructed and need a continuous maintenance which is particularly time consuming, which is unaffordable in large scale productions.